The present invention relates to an improved process for the preparation of acetaldehyde by carbonylation of methanol in the presence of hydrogen.
Acetaldehyde is an intermediate of great interest in the chemical industry. It is, in particular, useful for the manufacture of acetic acid and of acetic anhydride (cf. "Encyclopedia of Chemical Technology," Kirk-Othmer, 3rd edition, Vol. 1, pages 97 et seq.).
Industrial processes for producing acetaldehyde have been developed. The process most used at the present time is direct oxidation of ethylene. However, since this hydrocarbon originates from petroleum, it may become more economical to choose starting materials originating from synthesis gas, such as methanol.
The use of methanol as a starting material in the synthesis of products which traditionally are produced on an industrial scale from ethylene has formed the subject, and continues to be the subject, of numerous research projects. These research projects essentially concern the application of carbonylation techniques, that is to say, the reaction of carbon monoxide with methanol, where appropriate, in the presence of hydrogen.
Thus, numerous earlier investigations have concerned the homologisation of methanol, ie., the production of ethanol by carbonylation of methanol.
It is well known that methanol reacts with a 1 to 1 mixture of carbon monoxide and hydrogen at 185.degree. C., and under 360 atmospheres pressure, in the presence of dicobalt octacarbonyl. Under these conditions, a mixture of various products, containing ethanol, is obtained, the selectivity in respect of ethanol being relatively low (cf. Wender et al, "Science," Vol. 113, page 206, 1951).
Other authors have shown that when the homologisation reaction is carried out at 200.degree. C. under 200-350 atmospheres pressure in the presence of cobalt diacetate, the yield of ethanol can be improved by the simultaneous measures of operating in the presence of an iodine-containing promoter (I.sub.2 or CH.sub.3 I) and using a gas mixture rich in carbon monoxide (cf. Berty et al., "Chem. Tech.," Vol. 5, pages 260-266, 1956).
Further progress towards the production of ethanol has been achievable by addition of a methanol-soluble, phosphorus-containing compound to the preceding catalyst system (cf. French Pat. No. 1,341,840), by introducing ruthenium halides or osmium halides (cf. U.S. Pat. No. 3,285,948) or by addition of a tertiary phosphine and a hydrocarbon as a solvent (cf. French Pat. No. 2,314,166).
However, these processes are not applicable on an industrial scale. They do not make it possible to achieve high selectivities in respect of ethanol and, consequently, necessitate the setting up of complex installations for separating the various constituents of the mixture obtained, which unacceptably worsens the overall economics of such a process.
Starting from this observation, other investigators have concerned themselves with the production of acetaldehyde. Thus, it has been stated that the presence of an amount of cobalt of less than 2 millimols per mol of methanol in the catalyst system (cobalt/halogen) favors the conversion of methanol to acetaldehyde (cf. U.S. Pat. No. b 3,356,734).
In effect, if the carbonylation of methanol is carried out in accordance with the technique which forms the subject of the above-mentioned U.S. patent, at 185.degree. C. under 300-400 atmospheres pressure, with a CO/H.sub.2 molar ratio of 1.4, for 2 hours, about 130 g. of acetaldehyde per liter of reaction medium per hour are obtained, the productivity being of the order of 70 g. of acetaldehyde per hour per gram of cobalt involved in the reaction, allowing for the fact that the dimethoxyethane formed is a potential source of acetaldehyde
More recent work (cf. Japanese patent applications Nos. 77/136,111 and 77/133,914) has shown that the results achieved with this catalyst system can be substantially improved by adding to the system (cobalt/iodine) a substantial quantity of a phosphorus-containing compound or of a compound of arsenic, antimony, or bismuth. Nevertheless, the productivity in respect of acetaldehyde, relative to the cobalt employed, remains low, in site of the high pressures employed. Furthermore, in the known processes, a not insignificant part of the methanol is converted to butanol, butanal, and butenal, which are products for which demand is irregular, and which worsen the overall economics of the process by necessitating supplementary separation steps. Furthermore, the possibility of industrial exploitation of such processes is hampered by the high pressure required to achieve acceptable hourly productivities. Certain authors (cf. T. Mizoroki et al., "Bull. Chem. Soc. Jap.," Vol. 52(2), 479, 1979) have studied the carbonylation of methanol in the presence of cobalt and of methyl iodide, in methyl benzoate and under about 150 bars pressure. They have confirmed that under these conditions, the addition of small amounts of ruthenium substantially increases the selectivity in respect of ethanol, at the expense of acetaldehyde. They have also shown the overwhelming role played by sodium iodide in this reaction. Nevertheless, the hourly productivity of acetaldehyde does not exceed 65 g. per hour per liter of reaction medium and only reaches 14 g. per hour per gram of cobalt employed in the reaction.
It has now been found, totally unexpectedly, that the addition of small amounts of ruthenium to a catalyst system having a low cobalt content makes it possible considerably to increase the performance characteristics of the said system, thus permitting the selective production of acetaldehyde by hydrocarbonylation of methanol, with a remarkable hourly productivity, under a total pressure as low as 100 bars.
It is, therefore, an object of the present invention to provide a novel process for the homologisation of methanol to produce acetaldehyde in commercially practical yields.
It is also an object of the present invention to provide a process for producing acetaldehyde from methanol by homologisation which employs practical pressures.
Other objects will be apparent to those killed in the art from the present description.